I got my Mont Blanc rollerball some years ago when they decided to start making them. It was great, but then the quality of their refill cartridges became just awful. Waterman is an excellent choice--their basic rollerball cartridge writes very well, and you have a choice of a number of different styles. I'm not real familiar with the current line (my two, a LeMan 100 and an Opera, are a few years old), but I've used a Carene and it seemed very nice.
Haven't used my MB rollerball in a long time--possibly they've straightened out the reliability problems, but I haven't bothered to check.